1. Field of the Invention
The present invention relates to the field of digital data communication, and more particularly to wireless data communication to a plurality of subscribers or users in an interactive manner.
2. Prior Art
Various types of communication systems which include, at least in part, wireless data communication capabilities for communication to a large plurality of remote locations are known in the prior art. By way of example, in U.S. Pat. No. 4,117,405, a narrow band radio communication system is disclosed. In accordance with that disclosure, a narrow band system for communication between a transmitter device and a receiver device is provided by locking both devices to a carrier signal from a radio broadcast station and synthesizing a precise local frequency at leach device as a pre-set multiple of the frequency of the carrier. Such a system is usable in multi-station alarm and status communication including a multiplicity of independent radio alarm transmitters whose various carrier frequencies are different from, but phase locked to a local radio broadcast station, and a central alarm receiving station which employs a corresponding multiplicity of synchronized detectors. The detectors each have associated with them a synthesized local oscillator source which is also phase locked to the same radio broadcast station used by the alarm transmitters. Both the alarm transmitters and the central receiver are thereby accurately referenced to a readily available local frequency source, the broadcast station. As a consequence, very narrow band radio circuits may be employed and the receiver and transmitter band may be accurately matched to the information content of the alarm and status signal source to achieve high signal to noise ratio reliable transmissions. In U.S. Pat. No. 4,208,630, a narrow band paging or control radio system is disclosed which employs a similar synchronizing mechanism using a broadcast station for reference, though with the central transmitting apparatus transmitting signals to a multiplicity of remotely located receivers, which receivers may be used for paging purposes, or for controlling apparatus connected thereto.
The foregoing communication systems are generally directed toward the communication of identification codes, control signals, paging signals and the like of relatively limited information, but rather feature the capability of selective communication with any of a large number of remote receivers using the narrow band techniques disclosed therein. In other instances, it is desired to communicate large amounts of data to remote receivers so as to provide various types of information services for use in the home and by businesses alike. Such services may be either subscriber paid for, such as services which provide access to various types of data banks, electronic publishing or other information services, or services which are paid for by advertisers and the like, such as public transmission of catalogue and other advertising information.
Two types of services of this general type are known as videotex and teletex. Videotex uses the telephone line to provide two-way interactive communication so that the data requested by the user at the time is transmitted over the TV transmitter and/or the telephone line for display on the user's terminal or TV set. Such a system has the advantage of being interactive in that only the data desired by the user at the time need be transmitted, though it has the disadvantage of relatively low data communication rate capability if only the phone line is used in comparison to that normally desired, particularly in the forward mode, i.e., when communicating the requested data to the user. Teletex on the other hand is a one-way information transmission system using signals superimposed on television transmissions. The system has the advantage of high communication rate capabilities, though is highly limited because of the lack of interaction with the user. Consequently, usually teletex revenue is obtained mainly from advertisers as is customary in the television industry, while videotex revenues are expected to be derived primarily from subscribers. Obviously because of the advantages and disadvantages of each of these two systems, the nature of the information likely to be accessible over the two systems may be expected to be substantially non-overlapping, and accordingly both could be successful in the same geographic area. The purpose of the present invention however, is to provide a system having most of the advantages of both teletex and videotex in a single low cost system.